The present invention generally relates to systems and methods for cleaning workpieces. More specifically, the present invention relates to systems and methods for manipulating surface charges of cleaning tools, such as sponges, for improved cleaning of workpieces and of the cleaning tools.
Many manufacturing industries require systems for cleaning workpieces with highly polished surfaces. For example, workpieces such as semiconductor wafers, memory disks, compact disks, optical blanks, flat panel displays, read write heads and the like require thorough post-production cleaning. Often, this cleaning requires the removal of tiny particles, such as sub-micron and sub-half-micron sized particles, from a surface. Failure to thoroughly remove such particles typically results in a low yield of usable product, especially in the semiconductor industry. Illustrative examples, below, will refer specifically to the use of this invention in connection with the processing of semiconductor wafers, but such examples are presented merely to aid in understanding, and are not intended to limit in any way the generality or application of the invention.
During its fabrication, a workpiece may be subjected to various processing steps. For example, fabricating a semiconductor wafer typically involves the growth or deposition of insulating layers, the deposition of metal or other conductive layers, impurity doping, photolithographic patterning, planarization and the like. Successful fabrication of semiconductor wafers, like the fabrication of many other types of workpieces, requires extreme cleanliness of the wafers at various stages of the process. Thus, fabrication steps are often preceded and/or followed by cleaning steps.
One step in fabricating a semiconductor wafer involves planarization, such as chemical-mechanical planarization (xe2x80x9cCMPxe2x80x9d). After CMP, wafers are typically cleaned by a process of scrubbing with a cleaning pad, sponge or brush, to remove particulates, residue, and contaminants remaining on the surface of the wafer. A pad, typically in the form of a flat, circular pad, or a brush, typically in the form of a cylindrical roller, is often made of a spongy material such as polyvinyl acetate (xe2x80x9cPVAxe2x80x9d) or similar material. Pads or brushes may have bumps, ridges, or other projections on their surfaces, and may take any form convenient for aiding in the cleaning process. For the purposes of this specification, all such pads, sponges and brushes, whatever their shape or configuration, will be referred to as xe2x80x9csponges.xe2x80x9d
Post-CMP cleaning of semiconductor wafers is often accomplished by scrubbing the wafers with PVA sponges while adding a cleaning fluid, such as ammonium hydroxide solution or de-ionized (xe2x80x9cDIxe2x80x9d) water. Mechanical scrubbing with sponges works well for removing larger particles from the surface of a wafer, but is far less effective at cleaning ultra-fine particles, such as sub-micron and sub-half-micron sized particles. Such microscopic particles often cling to microscopic irregularities in the highly-polished surface of a wafer. Since these ultra-fine particles often possess an electric charge, they may also adhere to a wafer via electrostatic, Van der Waals forces. Conventional cleaning methods, which typically rely on mechanical forces to clean a wafer, often fail to remove these small, charged particles, thus resulting in a decreased yield on the wafer and a decreased value of the wafer.
After sponges are used to scrub a wafer, especially following CMP, particles often accumulate on, or are embedded into, the surface of the sponges. For example, conventional sponges typically have a slightly negative surface charge over a wide range of pH, so the surfaces of these sponges often accumulate positively charged, residual particles from the wafer surface. If not removed, these particles may damage the surfaces of subsequent wafers when the same sponges are used again for scrubbing. Semiconductor wafers are processed to fabricate devices having extremely fine detail. Thus, any scratching of a wafer can cause a device to fail, resulting in a decreased yield on the wafer. Conventional systems and methods for cleaning sponges typically involve washing the sponges with a cleaning fluid while oscillating or otherwise mechanically cleaning the brushes. These methods often fail to remove the small, charged particles described above. As residual particles accumulate on a sponge""s surface, the ability of the sponge to effectively clean wafers decreases. When a sponge becomes sufficiently saturated with particles, it no longer functions properly and must be discarded and replaced.
Thus, a need exists for systems and methods for effectively cleaning the highly-polished surfaces of semiconductor wafers and other workpieces. A need also exists for systems and methods for effectively cleaning sponges used to clean workpieces.
The present invention meets the aforementioned needs by providing systems and methods for cleaning workpieces and workpiece cleaning tools, such as sponges. In one embodiment, a system includes one or more workpiece cleaning stations. A workpiece cleaning station may include one or more sponges, a workpiece cleaning fluid, a sponge cleaning fluid and a fluid delivery system. Sponges may have positive, negative or neutral surface charges. Workpiece cleaning fluids and/or sponge cleaning fluids may be configured to alter the surface charges of one or more sponges. For example, a cleaning fluid may contain a sponge charge modifying agent, such as an aliphatic or aromatic amine. By manipulating the surface charge of one or more sponges, either during workpiece cleaning, during sponge cleaning or both, the systems and methods of the present invention provide more efficient cleaning of workpieces and of sponges.
According to one aspect of the invention, a method for cleaning workpieces and sponges is provided. In one embodiment, a workpiece may be cleaned with one or more sponges and a cleaning fluid and then removed from the sponges. After cleaning a workpiece, one or more sponges may be exposed to a sponge cleaning fluid, either in the same location or in a separate sponge cleaning apparatus. Either the workpiece cleaning fluid, the sponge cleaning fluid or both may be configured to alter the surface charge of one or more sponges. Thus, the surface charge of a sponge may be manipulated to more effectively attract or repel charged particles from the surface of a workpiece. The surface charge of a sponge may also be manipulated to more easily remove charged particles from its surface during sponge cleaning.